Many multimedia applications, such as news-on-demand, distance learning, and corporate training, rely on the efficient transfer of pre-recorded or live multimedia streams between a server computer and a client computer. These media streams may be captured and displayed at a predetermined rate. For example, video streams may require a rate of 24, 29.97, 30, or 60 frames per second. Audio streams may require 44,100 or 48,000 samples per second. An important measure of quality for such multimedia communications is the precisely timed playback of the streams at the client location.
Achieving this precise playback is complicated by the popular use of variable bit rate (VBR) media stream compression. VBR encoding algorithms allocate more bits per time to complex parts of a stream and fewer bits to simple parts, in order to keep the visual and aural quality reasonably uniform. For example, an action sequence in a movie may require more bits per second than the credits that are displayed at the end.
VBR compression may result in bursty network traffic and uneven resource utilization when streaming media. Additionally, due to the different transmission rates that may occur over the length of a media stream, transmission control techniques may need to be implemented so that a client buffer neither underflows or overflows. Transmission control schemes generally fall within one of two categories: they may be server-controlled or client-controlled.
Server-controlled techniques generally pre-compute a transmission schedule for a media stream based on a substantial knowledge of its rate requirements. The variability in the stream bandwidth is smoothed by computing a transmission schedule that has a number of constant-rate segments. The segment lengths are calculated such that neither a client buffer overflow nor an underflow will occur.
Server-controlled algorithms may use one or more optimization criteria. For example, the algorithm may minimize the number of rate changes in the transmission schedule, may minimize the utilization of the client buffer, may minimize the peak rate, or may minimize the number of on-off segments in an on-off transmission model. The algorithm may require that complete or partial traffic statistics be known a-priori.
Client-controlled algorithms may be used rather than server-controlled algorithms. In a client-controlled algorithm, the client provides the server with feedback, instructing the server to increase or decrease its transmission rate in order to avoid buffer overflow or starvation.